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The Locomotive Boiler.                        335

plate; but the tube-plate x, \\ inch thick, is throated to fit the
furnace flue. The top and side plates, J- inch thick, are in three
pieces, with joints at bbb, and wherever three thicknesses super-
pose, the mid plate is feathered, as at b. Screwed stays i£ inch
diameter, 7 inches apart, are fixed between the chambers at e and
at the sides, while the roof is supported by girder stays which
each consist of two plates resting by their ends on the roof seam.
Between these plates are passed collar bolts, which, after being
screwed into the roof and fastened by nuts, are tightened against
special washers on the girder. The furnace flues are of the Fox
pattern, flanged to the throat plate as shewn.

The Locomotive Boiler (Fig. 312) was the earliest form
of multitubular boiler, and has served as pattern for many other
steam generators. The firebox A is cubical and of -|" copper-
plate, thickened at: the tubes to ££". The back plate D is1 flanged,
and dished round j the firebox hole to the form shewn, the tube
plate c being also flanged. The top and sides are in one piece E,
and all these plates, being flat and weak, are supported from the
outer shell by screwed stays riveted over. The latter are £"
diam. and 4" pitch, and must be of copper, to avoid corrosion
by galvanic action, which frequently occurs next the firebox plate.
The shell top and sides are in one plate H, cut out as shewn at
HX ; the throat plate F is flanged to join the barrel and the firebox
•shell; and the back plate G is also flanged. The foundation ring a
serves as a distance or closing piece when fastening the shell to
the box, and a similar piece is required at z, called the firehole
ring. Mudhole bosses b b are welded on the solid plate, and
tapped for tapered screw plugs. A hole is cut in the top of the
shell at v for a double safety valve, and the plate stiffened by a
wrought-iron valve seating, From angles on the shell roof at w
are hung the sling stays x x, supporting the girder stays Y, the
latter being solid forgings, and the stay bolts taking the form of
tap bolts. T is a stiffening angle for the shell back, and p p are
expansion brackets which rest on the engine frame. The firebox
tube plate, besides thef ordinary screwed stays, has four palm stays
at s s, which are seen in detail at st. Two plates, K and j, form
the boiler barrel, and each makes a complete circle, the joints
being shewn in plan, well out of the water space. The dome L